(2) Operating System Structures

Operating Systems Structures 1

Common System Components

Operating-System Structures System Components Operating System Services System Calls System Programs System Structure Virtual Machines System Design and Implementation • System Generation • • • • • • •

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Process Management Main Memory Management File Management I/O System Management Secondary Management Networking Protection System Command-Interpreter System

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Process Management (cont.)

Process Management

• The operating system is responsible for the following activities in connection with process management

• A process is a program in execution – A process needs certain resources, including CPU time, memory, files, and I/O devices, to accomplish its task`

– Process creation and deletion – Process suspension and resumption – Provision of mechanisms for: • process synchronization • process communication 5

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Main-Memory Management

Main-Memory Management (cont.) • The operating system is responsible for the following activities in connections with memory management

• Memory is a large array of words or bytes, each with its own address – It is a repository of quickly accessible data shared by the CPU and I/O devices

• Main memory is a volatile storage device. It loses its contents in the case of system failure

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– Keep track of which parts of memory are currently being used and by whom – Decide which processes to load when memory space becomes available – Allocate and deallocate memory space as needed 8

File Management (cont.)

File Management

• The operating system is responsible for the following activities in connections with file management:

• A file is a collection of related information defined by its creator – Commonly, files represent programs (both source and object forms) and data

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I/O System Management • The I/O system consists of: – A buffer-caching system – A general device-driver interface – Drivers for specific hardware devices

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– File creation and deletion – Directory creation and deletion – Support of primitives for manipulating files and directories – Mapping files onto secondary storage – File backup on stable (nonvolatile)10 storage media

Secondary-Storage Management • Since main memory (primary storage) is volatile and too small to accommodate all data and programs permanently, the computer system must provide secondary storage to back up main memory • Most modern computer systems use disks as the principle on-line storage medium, for both programs and data 12

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Networking (Distributed Systems)

Secondary-Storage Management (cont.)

• A distributed system is a collection processors that do not share memory or a clock

• The operating system is responsible for the following activities in connection with disk management:

– Each processor has its own local memory

– Free space management – Storage allocation – Disk scheduling

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Networking (Distributed Systems) (cont.) • A distributed system provides user access to various system resources • Access to a shared resource allows:

• The processors in the system are connected through a communication network • Communication takes place using 14 a protocol

Protection System • Protection refers to a mechanism for controlling access by programs, processes, or users to both system and user resources • The protection mechanism must: – distinguish between authorized and unauthorized usage – specify the controls to be imposed – provide a means of enforcement

– Computation speed-up – Increased data availability – Enhanced reliability 15

Command-Interpreter System

Command-Interpreter System (Cont.) • The program that reads and interprets control statements is called variously:

• Many commands are given to the operating system by control statements which deal with: – Process creation and management – I/O handling – Secondary-storage management – Main-memory management – File-system access – Protection – Networking

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– command-line interpreter – shell (in UNIX)

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Its function is to get and execute the next command statement

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Operating System Services

Operating System Services (cont.)

• Program execution – system capability to load a program into memory and to run it • I/O operations – since user programs cannot execute I/O operations directly, the operating system must provide some means to perform I/O • File-system manipulation – program capability to read, write, create, and delete files 19

Additional Operating System Functions Additional functions exist not for helping the user, but rather for ensuring efficient system operations – Resource allocation – allocating resources to multiple users or multiple jobs running at the same time 21

System Calls

• Communications – exchange of information between processes executing either on the same computer or on different systems tied together by a network. Implemented via shared memory or

message passing • Error detection – ensure correct computing by detecting errors in the CPU and memory hardware, in I/O devices, or in user programs 20

Additional Operating System Functions (cont.) – Accounting – keep track of and record which users use how much and what kinds of computer resources for account billing or for accumulating usage statistics – Protection – ensuring that all access to system resources is controlled

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System Calls (cont.) • Three general methods are used to pass parameters between a running program and the operating system

• System calls provide the interface between a running program and the operating system – Generally available as assemblylanguage instructions – Languages defined to replace assembly language for systems programming allow system calls to be made directly (e.g., C, C++) 23

– Pass parameters in registers – Store the parameters in a table in memory, and the table address is passed as a parameter in a register – Push (store) the parameters onto the stack by the program, and pop off the stack by operating system 24

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Passing of Parameters As A Table

Types of System Calls • • • • •

Process control File management Device management Information maintenance Communications

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Communication Models • Communication may take place using either message passing or shared memory

Message Passing

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System Programs • System programs provide a convenient environment for program development and execution. The can be divided into:

Shared Memory

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System Programs (cont.)

– File manipulation – Status information – File modification – Programming language support – Program loading and execution – Communications – Application programs

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MS-DOS System Structure

• Most users’ view of the operation system is defined by system programs, not the actual system calls

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• MS-DOS – written to provide the most functionality in the least space – Not divided into modules – Although MS-DOS has some structure, its interfaces and levels of functionality are not well separated

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UNIX System Structure

UNIX System Structure (cont.) –The kernel

• UNIX – limited by hardware functionality, the original UNIX operating system had limited structuring. The UNIX OS consists of two separable parts

• Consists of everything below the system-call interface and above the physical hardware • Provides the file system, CPU scheduling, memory management, and other operating-system functions; a large number of functions for one level

– Systems programs – The kernel 31

Layered Approach

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An Operating System Layer

• The operating system is divided into a number of layers (levels), each built on top of lower layers. The bottom layer (layer 0), is the hardware; the highest (layer N) is the user interface. • With modularity, layers are selected such that each uses functions (operations) and services of only lower-level 33 layers

Modules

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Solaris Modular Approach

• Most modern operating systems implement kernel modules – Uses object-oriented approach – Each core component is separate – Each talks to the others over known interfaces – Each is loadable as needed within the kernel

• Overall, similar to layers but with more flexible 35

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Virtual Machines (cont.)

Virtual Machines • A virtual machine takes the layered approach to its logical conclusion. It treats hardware and the operating system kernel as though they were all hardware • A virtual machine provides an interface identical to the underlying bare hardware

• The operating system creates the illusion of multiple processes, each executing on its own processor with its own (virtual) memory

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Virtual Machines (Cont.)

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System Models

• The resources of the physical computer are shared to create the virtual machines – CPU scheduling can create the appearance that users have their own processor – Spooling and a file system can provide virtual card readers and virtual line printers – A normal user time-sharing terminal serves as the virtual machine 39 operator’s console

Advantages/Disadvantag es of Virtual Machines • The virtual-machine concept provides complete protection of system resources since each virtual machine is isolated from all other virtual machines. This isolation, however, permits no direct sharing of resources.

Non-virtual Machine

Virtual Machine

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Advantages/Disadvanta ges of Virtual Machines (cont.) • A virtual-machine system is a perfect vehicle for operatingsystems research and development. System development is done on the virtual machine, instead of on a physical machine and so does not disrupt normal system operation.

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Advantages/Disadvanta ges of Virtual Machines (cont.) • The virtual machine concept is difficult to implement due to the effort required to provide an exact duplicate to the underlying machine

System Design Goals • User goals – operating system should be convenient to use, easy to learn, reliable, safe, and fast • System goals – operating system should be easy to design, implement, and maintain, as well as flexible, reliable, error-free, and efficient

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Mechanisms and Policies

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System Implementation

• Mechanisms determine how to do something, policies decide what will be done • The separation of policy from mechanism is a very important principle, it allows maximum flexibility if policy decisions are to be changed later

• Traditionally written in assembly language, operating systems can now be written in higher-level languages • Code written in a high-level language: – Can be written faster – Is more compact. – Is easier to understand and debug

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System Implementation (cont.) • An operating system is far easier to port (move to some other hardware) if it is written in a high-level language

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System Generation (SYSGEN) • Operating systems are designed to run on any of a class of machines; the system must be configured for each specific computer site • SYSGEN program obtains information concerning the specific configuration of the hardware system

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System Generation (SYSGEN) (cont.) • Booting – starting a computer by loading the kernel • Bootstrap program – code stored in ROM that is able to locate the kernel, load it into memory, and start its execution

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